Films with high optical density (OD) of >3.5 or, respectively, very low transparency of <0.1% are needed by way of example for blinds in the greenhouse sector, where these blinds inhibit penetration of daylight and thus permit establishment of different day/night periods by using artificial light, irrespective of ambient light. These blinds also prevent impairment of the environment of the greenhouse by strong artificial light during the night. Another possible important use is found in the sector of roller blinds and Venetian blinds, where the intended use of these is for reduction of light levels. Films of this type are also suitable for use in projection screens and as films in display applications, for example for mobile telephones.
However, considerable further requirements are placed upon the film in particular by the first two application sectors (blinds, including roller blinds) and the last application sector (displays). By way of example, the thickness of these cannot be greater than 60 μm, otherwise they are insufficiently flexible for the final applications, and the film also has to have a certain UV resistance in order to be suitable for long-term use. In display applications, the total thickness of the film is an important factor for the final thickness of the display, which is intended to have minimum thickness.
At least in the two first-mentioned applications, it is also desirable that the film has low flammability and ideally is self-extinguishing in the absence of any supportive fire, in order that it does not contribute to spread of fire in greenhouses and other buildings.
The films must also be white on at least one side, in order to reflect a maximum portion of the incident light, for reasons which include avoidance of heating of the material itself and contribution to energy efficiency. High whiteness in reflected light or, respectively, high lightness in reflected light is also important.
The film is also intended to be capable of cost-effective production on existing multilayer polyester film plants. These generally comprise, in the thickness range up to 75 μm, a main extruder suitable for high throughputs, and from one to two extruders for the outer layers, suitable for markedly lower throughputs.
Films that are white on one side and that have high optical density have been described by way of example in U.S. Pat. No. 6,436,219.
Although U.S. Pat. No. 6,436,219 gives a general description of films of thickness from 15 to 350 μm, the thicknesses of the films in the examples are all 80 μm.
As the films become thicker, it becomes easier to achieve high whiteness, which makes it more difficult to perceive the black outer layer. It also becomes easier to achieve high optical density (Lambert-Beer law). However, the specification does not reveal any production method for films with high whiteness and high optical density with total thickness markedly smaller than 80 μm. Nor does it reveal any information about achievement of suitable UV resistance and optional achievement of suitable flame retardancy.